This invention relates generally to fans for moving air and, more particularly, to an improved outlet guide vane design for axial flow fans.
Axial flow fans are used in a wide range of applications, including HVAC, refrigeration, automotive, power systems and aerospace. Important considerations for these applications include efficiency, noise level, operating range, compactness, reliability and cost.
High performance axial flow fans typically utilize stationary outlet guide vanes to recover swirl flow generated by the upstream fan blades. This recovery process involves the transformation of swirl kinetic energy into increased static pressure across the guide vanes, and leads to significant improvement in efficiency. To achieve effective performance, care must be taken to design the vanes to be well aligned with the oncoming swirl and to ensure that they are able to turn the flow back to the axial direction with minimal total pressure loss.
The outlet guide vanes in this type of fan extend spanwise from inner to outer casing walls. Several equally spaced vanes are normally used; each is generally identical in shape and cambered to have a concave pressure surface and a convex suction surface. Each of those surfaces extends between the vane leading and trailing edges. The vanes are typically defined by generating a series of airfoil profiles along a spanwise stacking line. The various profiles may vary in thickness, camber and chord length, and the spanwise stacking line may take a variety of forms including those with bowed shapes, circumferential lean and axial sweep.
It is common practice to design the guide vanes so as to properly address the specific localized flows associated with a particular fan design. That is, vanes are generally optimized for spanwise flow variations by collectively varying the vane twist, camber and chord parameters. In addition, the vanes may be leaned in the circumferential direction or swept axially to “de-phase” the interaction of the fan blade wakes with the guide vanes, resulting reduced noise level.
In addition, a variety of methods for reducing total pressure losses associated with vane end-wall effects have been invented. These methods have been generally intended for use in the multi-stage compression section of gas turbine engines. One related concept is that shown in U.S. Pat. No. 2,795,373 issued to Hewson et. al., entitled “Guide Vane Assemblies In Annular Fluid Ducts.” That patent proposes to reduce vane end-wall losses by using vanes having a curved stacking line or a stacking line composed of two angled sections that meet at the vane mid-span station.
Another technique is described in U.S. Pat. No. 5,088,892 issued to Weingold et. al., entitled “Bowed Airfoil for the Compression Section of a Rotary Machine.” That patent shows an airfoil wherein the spanwise stacking line is straight over the mid-section of the airfoil and angled circumferentially in the end wall regions. The intent is similar to Hewson in managing vane losses in the vicinity of the end walls, but with a stiffer and lighter vane design.
Of particular interest in the present invention is the strong swirl flow that is produced in the clearance region between the fan blade tips and the casing wall. This localized swirl is especially important in fans with low ratio of axial flow velocity relative to blade tip speed (low flow coefficient), and can produce excessive loading and flow separation in the outboard stations of the stator vanes. Use of conventional guide vane design in such cases produces reduced fan efficiency and limitations in operating range.
The applicant has found that variations beyond the above art can be made to obtain further improvements in controlling the flow separation in axial flow fan outlet guide vanes, particularly for fans operating at low flow coefficients. These improvements have been developed by performing three-dimensional computational fluid dynamic analysis on an extensive series of fan rotor and vane design combinations. The performance benefits and vane stall properties have also been verified experimentally.